Results of long-term follow-up after compensated fixed-dose therapy for thyrotoxicosis

Technical observations on the assessment of thyroid volume by palpation and ultrasonography

OBJECTIVE

The diagnosis of increased thyroid volume, in field studies of goiter prevalence, has been based on inspection and palpation of the thyroid. Because clinical examination, compared with ultrasonography, has a low positive predictive value for the presence of a goiter, it overestimates goiter prevalence. It also has the problem of marked interobserver variability. This led to the use of ultrasonographic scanners in field studies. The problem with the latter is the cost and skill required for the complicated linear measures and their translation to lobe volume and then thyroid volume. We studied patients to determine whether this complicated assessment could be simplified.

METHODS

We studied palpation in 31 patients with thyroid disease in whom individual ultrasonographic linear dimensions were also obtained in their 62 thyroid lobes to determine their relationship to thyroid lobe volume.

RESULTS

Palpation revealed poor discrimination of smaller thyroid sizes as determined by ultrasonography. Stepwise linear regression (backward selection) revealed that of the 3 thyroid dimensions, only the lateromedial dimension of the thyroid lobe had a significant correlation to lobe volume, accounting for 82.5% of the variability in lobe volume. The lobe volume (in milliliters) is given by the lobe lateromedial dimension (in centimeters) multiplied by 13 minus a constant of 15.

CONCLUSIONS

A simple linear ultrasonographic measurement of the thyroid lateromedial dimension, which can be done with little training, is as good as more complicated measures of thyroid volume estimation by ultrasonography and is an ideal method for identifying goiters in field surveys.

A mathematical model of optimized radioiodine-131 therapy of Graves' hyperthyroidism

BACKGROUND: The current status of radioiodine-131 (RaI) dosimetry for Graves' hyperthyroidism is not clear. Recurrent hyperthyroidism and iatrogenic hypothyroidism are two problems which interact such that trying to solve one leads to exacerbation of the other. Optimized RaI therapy has therefore begun to be defined just in terms of early hypothyroidism (ablative therapy) as physicians have given up on reducing hypothyroidism. METHODS: Optimized therapy is evaluated both in terms of the greatest separation of cure rate from hypothyroidism rate (non-ablative therapy) or in terms of early hypothyroidism (ablative therapy) by mathematical modeling of outcome after radioiodine and critically discussing the three common methods of RaI dosing for Graves' disease. RESULTS: Cure follows a logarithmic relationship to activity administered or absorbed dose, while hypothyroidism follows a linear relationship. The effect of including or omitting factors in the calculation of the administered I-131 activity such as the measured thyroid uptake and effective half-life of RaI or giving extra compensation for gland size is discussed. CONCLUSIONS: Very little benefit can be gained by employing complicated methods of RaI dose selection for non-ablative therapy since the standard activity model shows the best potential for cure and prolonged euthyroidism. For ablative therapy, a standard MBq/g dosing provides the best outcome in terms of cure and early hypothyroidism.

Late and transient increases in free T4 after radioiodine treatment for Graves' disease

The objective of this retrospective study was to evaluate the fall in free T4 (FT4) in patients with Graves' disease after treatment with radioiodine in a fixed dose of 600 MBq. The study was performed at our outpatient clinic with patients referred from primary care during the time period January 1989 to January 1995. Only patients not given anti thyroid drugs after radioiodine were included. FT4 and TSH were measured every second week for the first three months, and thyroxine substitution started when the FT4 was at or below 15 pmol/l. Of the 60 patients thus available for evaluation, 7 required retreatment, giving a "success rate" of 88%. Of the 53 patients successfully treated with one dose of radioiodine, 36 had not been pretreated with anti thyroid drugs. Among these patients 13 (36%) had a transient increase in FT4 after radioiodine therapy, which mostly occurred after 4 to 6 weeks. The remaining 17 patients had been given carbimazole prior to radioiodine. In this group 8 (53%) had a transient increase in FT4, generally after 2 weeks. In conclusion, giving a fixed large dose of radioiodine and starting thyroxine substitution before hypothyroidism has developed is a workable clinical routine. Although a gradual fall in FT4 was the rule, a transient increase in FT4 was noticed in 30-50% of the patients 2 to 6 weeks after treatment.